Delta

Delta

This entry is compiled and applied by "popular science China" Science Encyclopedia

to examine

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Delta

Delta, the estuarine alluvial plain, is a common surface topography. The estuary is the end of a river, that is, the place where the river flows into the sea, lake or other rivers. The inland river that does not flow into the lake is called tailless River, which can have no estuary. The section of the estuary is enlarged, and the flow velocity is decreased sharply. A large amount of sediment is deposited to form a delta, which is called Delta. The top of the delta points to the upper reaches of the river, and the outer edge faces the sea, which can be regarded as the "bottom" of the triangle.

Delta is delta in English, and delta also means the fourth letter of the Greek alphabet. The Greek letter Δ is similar to the shape of a delta, which some people think is the pictographic origin of Δ.

brief introduction

Delta is a sediment depositional system supplied by river

It is the product of the interaction of "dynamics sedimentation geomorphology" and other factors, distributed in the areas where rivers flow into the ocean or lakes. Its plane shape is mostly triangular, with the top pointing upstream and the bottom facing the open sea. It is made up of land and underwater parts.

The basic conditions for the formation of the delta mainly depend on the basic law of the flow change in the estuary area. Therefore, the river, marine structure, climate and basin physical geography and other factors have a profound impact on the sedimentary characteristics and morphological types of the delta. According to the different geomorphic position, the delta is generally divided into the top layer, the front layer and the bottom layer from the continent to the sea

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Geographical features

Delta is an alluvial plain formed by a large amount of sediment carried by a river when it flows into the ocean, lake or other rivers due to the decrease of flow velocity. The delta formed by confluence of lakes and rivers is also called Inland River Delta. There are some rivers directly alluvial Delta on land, but not into other water systems, such delta is called inland delta, is a kind of Inland River Delta.

Delta, also known as estuarine plain, looks like a triangle from the plane. The top points to the upstream and the bottom is its outer edge. Therefore, it is called Delta. The Delta has a large area, deep soil layer, dense water network, flat surface, fertile soil and prone to flood.

geographical distribution

Most of the world's major rivers enter the sea with a delta. For example, the Nile River in Egypt (the world's longest river) has a huge delta with an area of 24000 square meters; the Mississippi River in the United States (the world's fourth longest river) has a bird foot like delta with an area of 26000 square meters; the Yangtze River (the world's third longest river), the Yellow River (the world's fifth longest river) and the Pearl River in China also have large deltas.

Geological advantages

Delta area is generally low and flat with dense river network, so it is mostly good farming area. For example, China's Pearl River Delta, Yangtze River Delta and other estuaries are all agricultural high-yield areas. Although the land in the Yellow River Delta is fertile, due to the influence of saline alkali, the farming conditions are slightly worse.

The Delta has a large area, deep soil layer, dense water network, flat surface and fertile soil. It is different from the fan-shaped alluvial plain near the foothills. The fan-shaped alluvial plain has smaller area, thinner soil layer, gravel texture and less fertile soil than the delta.

Delta area is not only a good farming area, but also favorable for the formation of oil and gas. Many famous oil and gas fields in the world are distributed in the delta area.

Formation principle

Formation of estuarine delta

It is the result of a large amount of sediment accumulation in the estuary when the river action exceeds the water action. When the alluvial deposits accumulate in the estuary, a series of underwater shoals, core shoals or sandbanks appear first, and the current bifurcates and forms an underwater delta inclined to the sea. With the growth and decline of the crossroads and the merging and expansion of the central beach, the front edge of the subaqueous delta continuously advances to the sea, while the rear edge becomes a part of the Subaqueous Delta because of the silting up of the beach and the covering of the flood deposits. Due to the continuous change of the crossroads, many intertwined riverside sand banks and lake marsh depressions are often formed in the delta.

About 16 billion cubic meters of sediment are transported into the sea by rivers every year in the world. When the sediment mixed in the river flows from the upstream to the downstream, due to the gradual expansion of the riverbed and the reduction of the drop, when the river flows into the sea, the flow is dispersed and the flow rate suddenly decreases. In addition, the tidal water flows in from time to time, which has the effect of blocking the river. In particular, there are many strong ionizing sodium chloride (salt) dissolved in the sea, which produces a large number of ions, which can make the suspended sediment in the water It also precipitates. As a result, the sediment accumulates here and finally comes out of the water. At this time, the river had to bypass the sand and flow from both sides. Because the upstream surface of the sand pile is directly impacted by the river and constantly eroded by the running water, it often forms a tip shape, while the downstream surface is relatively wide, making the sand pile into a triangle, so people named them "delta".

In the estuarine area with strong wave action, the river flows into the sea in a single stream or only crosses in a small scale. In this case, only the sediment at the outlet of the main stream exceeds the erosion of the wave, which makes the delta elongate outwards in a sharp shape with the main stream as the center. The Yangtze River Delta belongs to this type.

Dynamic characteristics

In view of the dynamic characteristics of the estuary area, in the range of the tidal current boundary moving up and down, because the river is supported by the tidal current and the flow velocity is small, it is easy to form the central beach and the central bar, which makes the river bifurcate. At the mouth of the estuary, due to the diffusion of water and the slow down of flow velocity, the sediment often accumulates into shoals and resists the estuary, so it is called bar sand, which is the main obstacle to navigation in the estuary area.

fluid mechanics

Bates (1953) studied the hydrodynamics of delta formation. He compared the delta estuary to a nozzle in hydraulics. When the river flows into the reservoir through the estuary, it forms a free jet, which can be divided into two types: axial jet and plane jet. 1. Axial jet: the mixing action of river water and reservoir water occurs in three-dimensional space (longitudinal), and its mixing action is faster, resulting in the rapid decrease of water flow speed. 2. Plane jet: it means that the mixing of river water and reservoir water occurs in two-dimensional space (surface water), and its mixing is slow, but it can spread to a far place in the basin. The response of axial jet and plane jet in marine delta and lake delta is different.

(1) Equal density flow pattern: the density of river water and water storage body is nearly equal (lake delta type). The density of river water and water storage body is equal when they are injected into the lake, and the axial jet occurs, but the plane jet is not obvious. The water is mixed in three-dimensional space, the velocity of flow decreases rapidly, and the sediment accumulates rapidly in the estuary, forming a lake delta.

(2) Low density flow pattern: the density of inflow water is less than that of reservoir (marine delta type), the density of river water (fresh water) is less than that of sea water (salt water), and the density of river water is 6% of that of sea water. The fresh water flows outwards on the salty water surface, mainly by plane jet. The sediments spread far out, that is to say, the sediments can be brought to the far shore to deposit, forming a coastal delta dominated by fluvial action.

(3) High density flow pattern: the density of inflow water is higher than that of storage body. Gravity flow pattern: the density of inflow water is higher than that of storage body. A flat jet along the bottom of the water. When cold water flows into a warmer lake, or when flood water with a large suspended load enters the lake. The density of river water with silt is seldom higher than that of sea water. The high density flow pattern also occurs on the continental slope. The unconsolidated seabed sediment collapses due to the action of gravity, resulting in the formation of turbidity current.

Formation and development factors

The main controlling factors of delta formation and development are as follows: (1) flow velocity, water discharge and sediment load. (2) the relative density of discharge and impoundment is large. (3) the type and intensity of sedimentary medium (river, wave, tide and current). (4) the structural properties of sedimentary basin, including the stability of sedimentary basin, subsidence velocity and seawater advance and retreat.

Deltas are mainly formed by the accumulation of mud and sand transported by the river near the estuary, but they can be scoured, reformed and redistributed by waves or tides.

If the river is relatively large, carrying more sediment and the sea water is weak, the Delta will develop rapidly and continue to advance towards the sea (such as the Mississippi Delta in the United States).

If the river is small, the sediment transported is small, and the seawater action is strong, the delta develops slowly, or even does not develop Delta, but appears in estuarine environment (such as Hangzhou Bay and Qiantang River). The Mississippi Delta in the United States has a long river source and carries a large amount of mud and sand. The wave and tide action in the Gulf of Mexico is weak. As a result, the Mississippi Delta extends 48km to the Gulf of Mexico, becoming a rare Delta in the world.

The formation and development of the delta is essentially a process of branching and advancing towards the sea. In the estuary, due to the slowing down of the seabed slope, the dispersion of water flow and the sudden decrease of flow velocity, a large amount of bottom load material will accumulate and form the estuary bar.

Due to the blocking of water and sediment by the sand bar, the water flows to both sides

Chinese PinYin : San Jiao Zhou